For anyone looking for a grill cover for their Weber Spirit 310 from summer, this seems to fit the bill nicely and is within a few pennies of its lowest price ever. After reading a lot of terrible reviews of grill covers, including expensive ones directly from Weber, this seems like the best bang for the buck.
58 x 26 x 46
https://www.amazon.com/gp/product/B071YMTDBN